1. Field of the Invention
The present invention relates to the establishment of a fluid tight seal between a pair of relatively moveable parts between the inner diameter of a bore provided in a housing and a movable shaft which extends through that bore for example, and particularly to improving the performance of stuffing-box type packings which employ at least one packing ring which comprises expanded graphite. More specifically, this invention is directed to enhancing the service life of a sealing device which employs a packing ring comprising expanded graphite, the packing ring contacting a movable member. Accordingly, the general objects of the present invention are to provide novel and improved apparatus and methods of such character.
2. Description of the Prior Art
It is well known in the art to employ stuffing-box packings to prevent the leakage of fluid about movable shafts such as, for example, rotatable actuating spindles for valves. Such packings will often employ a packing ring which comprises expanded graphite, the packing ring contacting either or both of the spindle and the bore in the valve housing through which the spindle extends. It is also known to employ, in such packings, stripper elements which cooperate with the housing bore and/or the spindle. Such stripping elements are typically disc-shaped and may be comprised of plastic or metal.
Expanded graphite, particularly at relatively low strength, is known to have a tendency to adhere to surfaces. Accordingly, in the environment of a stuffing box packing there is a risk of graphite particles becoming separated from the packing ring and remaining adhered to the opposite surfaces which are to be sealed by the packing, i.e., the actuating spindle and the housing bore. Any such adhering particles may, on further rotary movement of the spindle, cause damage to the surface of the packing ring and thus compromise its ability to perform the requisite sealing function. The same problem, incidentally, is presented by other soft sealing materials of high adhesion and low strength. For most applications of stuffing-box packings, the adherence of particles of graphite or the like to the surface of a movable member about which a seal must be established does not present an unacceptable operating condition. There are, however, critical applications where only the most minimal degradation of the seal can be tolerated. As an example of such an application, environmental regulations may require that the escape of toxic media into the atmosphere be reliably prevented. In such instances, even the extreme of placing the sealing device within a bellows fails to afford adequate protection because, should there be a rupture of the bellows, a major release of toxic material could occur.
An example of a prior art stuffing-box packing which includes an expanded graphite packing ring and disc-shaped stripper elements may be seen from published German Application No. 3,839,792. In the packing of this published application, the component which establishes the seal is comprised of annular "foil" discs of expanded graphite and metal which have been stacked alternately and pressed together. Annular "foil" discs comprised of a metal fabric or plastic fabric and filled with a lubricant can additionally be interposed between the discs of expanded graphite and metal to enhance performance. Such interposed annular fabric discs, while primarily provided for the purpose of lubrication, also perform the function of stripping graphite particles which are detached from the graphite disc due to the low strength of expanded graphite. However, the stripping action which results does not affect the entire zone which is occupied by the expanded graphite lamellae and thus is inadequate. Restated, in a packing of the type disclosed in published German Application Number 3,839,792, even when fabric discs are interposed between the outer metal discs and the disc comprised of expanded graphite, the problem of seal degradation is not solved because no continuous stripping edges are provided and, accordingly, there is a risk that graphite particles will remain adhered to a surface which is designed to move relative to the graphite disc, i.e., the surface of a valve actuating spindle, and the seal will in time become impaired.
U.S. Pat. No. 3,039,780 discloses a different type of shaft seal wherein a pair of rubber sealing rings are arranged obliquely relative to the axis of the shaft about which a seal is to be provided. These rubber sealing rings are covered, on their end faces, by two porous, yielding felt discs. The felt discs serve to support the sealing rings and an oil-receiving chamber is located therebetween so as to provide lubrication.